Hydrogen separator and process for production thereof

ABSTRACT

A hydrogen separator comprising a porous substrate composed mainly of a ceramic having a large number of pores connecting from one surface of the substrate to other surface, and a hydrogen-separating layer made of a hydrogen permselective metal formed on the porous substrate via an intermediate layer made of an electron-conductive ceramic. The hydrogen separator hardly generates defects such as peeling, cracks or the like in the hydrogen-separating layer and is suitable for use even when the hydrogen separator is exposed to a heat cycle, used under high temperature conditions or/and used for long-term.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a hydrogen separator. Moreparticularly, the present invention relates to a hydrogen separatorwhich is suitable for use under high temperature conditions or/and forlong-term use, as well as to a process for producing such a hydrogenseparator.

A hydrogen separator comprising a porous ceramic substrate and ahydrogen-separating layer, such as palladium or palladium alloy, formedon the substrate has been used in order to separate only hydrogenselectively from a hydrogen-containing gas, such as steam-reformed gas.Such a hydrogen separator is used for hydrogen separation athigh-temperatures in some cases. Therefore, the hydrogen separator isrequired to have high gas tightness at high temperatures or during heatcycle in which temperature increase and decrease are repeated.

Conventional hydrogen separators have, as shown in FIG. 2, a structurecomprising a porous substrate 12 and a hydrogen-separating layer 13formed on one surface 15 of the porous substrate 12. Incidentally, theporous substrate 12 has a large number of pores connecting from onesurface 15 of the substrate to other surface (not shown), and isconstituted by a material such as ceramic or metal. In producing ahydrogen separator 11 having a structure such as shown in FIG. 2, forexample, there can be mentioned a method of plating palladium (whichbecomes a hydrogen-separating layer 13) on one surface 15 of a poroussubstrate 12 [see, for example, Patent Document 1 (JP-A-3-146122),Patent Document 2 (Japanese Patent No. 3213430) and Patent Document 3(JP-A-62-273030)].

When palladium layer has been formed directly on a porous ceramicsubstrate, however, there is a problem that the affinity between thepalladium and the porous ceramic substrate is not good. As a result,when the hydrogen separator obtained is exposed to a heat cycle, theremay happen peeling of palladium from porous substrate or generation ofother defects in palladium layer, and gas tightness of the palladiumlayer may reduce. Further, when, after the formation of palladium layeron a porous substrate, other metal, such as silver, is formed on thepalladium and then heated to form a palladium alloy as ahydrogen-separating layer. There is a problem that thehydrogen-separating layer made of the alloy tends to peel from theporous substrate more easily, because of additional heating foralloying.

SUMMARY OF THE INVENTION

The present invention has bee made in view of the above-mentionedproblems of prior art. The present invention aims at providing ahydrogen separator whose hydrogen-separating layer hardly generatesdefects such as peeling, cracks or the like and which is suitable foruse even when the hydrogen separator is exposed to a heat cycle, usedunder high temperature conditions or/and used for long-term; and aprocess for producing such a hydrogen separator.

The present inventors made a study in order to achieve the above aim. Asa result, it was found that the above aim is achievable by using,between the porous substrate and hydrogen-separating layer of hydrogenseparator, an intermediate layer made of an electron-conductive ceramichaving high affinity to both of the porous substrate andhydrogen-separating layer. The present invention has been completedbased on the finding.

According to the present invention, there are provided a hydrogenseparator and a process for production thereof, both described below.

-   [1] A hydrogen separator comprising:

a porous substrate composed mainly of a ceramic having a large number ofpores connecting from one surface of the substrate to other surface, anda hydrogen-separating layer made of a hydrogen permselective metalformed on the porous substrate via an intermediate layer made of anelectron-conductive ceramic.

-   [2] A hydrogen separator according to [1], wherein the    electron-conductive ceramic is at least one member selected from the    group consisting of titanium nitride (TiN), zirconium nitride (ZrN),    indium-tin oxide (In₂O₃.Sn), zinc oxide (ZnO) and titanium oxide    [TiO_(x)(0<x<2)].-   [3] A hydrogen separator according to [1] or [2], wherein the    ceramic for a porous substrate is at least one member selected from    the group consisting of alumina, silica, silicα-alumina, mullite,    cordierite and zirconia.-   [4] A hydrogen separator according to any of [1] to [3], wherein the    hydrogen permselective metal is palladium (Pd) or an alloy    containing palladium (Pd).-   [5] A hydrogen separator according to any of [1] to [4], wherein the    hydrogen-separating layer has a thickness between 1 and 5 μm.-   [6] A hydrogen separator according to any of [1] to [5], wherein an    anti-oxidant layer is formed on the intermediate layer in an inert    atmosphere or under a vacuum condition and the hydrogen-separating    layer is formed via the anti-oxidant layer.-   [7] A hydrogen separator according to [6], wherein the anti-oxidant    layer is a layer made of at least one metal selected from the group    consisting of palladium (Pd), platinum (Pt), silver (Ag) and gold    (Au).-   [8] A process for producing a hydrogen separator comprising a porous    substrate composed mainly of a ceramic having a large number of    pores connecting from one surface of the substrate to other surface,    and a hydrogen-separating layer made of a hydrogen permselective    metal formed on the porous substrate, wherein an intermediate layer    made of an electron-conductive ceramic is formed on the one surface    of the porous substrate, and then the hydrogen-separating layer is    formed on the surface of the intermediate layer.-   [9] A process for producing a hydrogen separator according to [8],    wherein the electron-conductive ceramic is at least one member    selected from the group consisting of titanium nitride (TiN),    zirconium nitride (ZrN), indium-tin oxide (In₂O₃.Sn), zinc oxide    (ZnO) and titanium oxide [TiO_(x)(0<x<2)].-   [10] A process for producing a hydrogen separator according to [8]    or [9], wherein the ceramic for a porous substrate is at least one    member selected from the group consisting of alumina, silica,    silicα-alumina, mullite, cordierite and zirconia.-   [11] A process for producing a hydrogen separator according to any    of [8] to [10], wherein the hydrogen permselective metal is    palladium (Pd) or an alloy containing palladium (Pd).-   [12] A process for producing a hydrogen separator according to any    of [8] to [11], wherein the hydrogen-separating layer has a    thickness between 1 and 5 μm.-   [13] A process for producing a hydrogen separator according to any    of [8] to [12], wherein an anti-oxidant layer is formed on the    intermediate layer in an inert atmosphere or under a vacuum    condition and the hydrogen-separating layer is formed via the    anti-oxidant layer.-   [14] A process for producing a hydrogen separator according to [13],    wherein the anti-oxidant layer is a layer made of at least one metal    selected from the group consisting of palladium (Pd), platinum (Pt),    silver (Ag) and gold (Au).

The hydrogen separator of the present invention hardly generates defectssuch as peeling, cracks or the like in the hydrogen-separating layer andis suitable for use even when the hydrogen separator is exposed to aheat cycle, used under high-temperature conditions or/and used forlong-term. According to the present process for producing a hydrogenseparator, there can be easily produced a hydrogen separator whichhardly generates defects such as peeling, cracks or the like in thehydrogen-separating layer and is suitable for use even when the hydrogenseparator is exposed to a heat cycle, used under high-temperatureconditions or/and used for long-term.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically showing an embodiment of thehydrogen separator of the present invention.

FIG. 2 is a sectional view schematically showing an embodiment ofconventional separators.

FIG. 3 is a sectional view schematically showing a state in which anintermediate layer has been formed on a porous substrate.

In these referential numerals, 1 and 11 are each a hydrogen separator; 2and 12 are each a porous substrate; 3 and 13 are each ahydrogen-separating layer; 5 and 15 are each one surface of a poroussubstrate; and 10 is an intermediate layer.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the present invention is described below.However, the present invention is not restricted to the followingembodiment and it should be understood that alterations, modificationsand etc. can be appropriately added to the following embodiment based onthe ordinary knowledge, as long as there is no deviation from the gistof the present invention.

FIG. 1 is a sectional view showing an embodiment of the hydrogenseparator of the present invention. As shown in FIG. 1, the hydrogenseparator 1 of this embodiment comprises a porous substrate 2 having alarge number of pores connecting from one surface 5 of the poroussubstrate 2 to other surface (not shown) and a hydrogen-separating layer3 formed on the porous substrate 2 via an intermediate layer 10. Theporous substrate 2 is composed mainly of a ceramic. Thehydrogen-separating layer 3 is made of a hydrogen permselective metal,and can selectively transmit only hydrogen from a hydrogen-containinggas (a to-be-treated gas) incoming from the one surface 5 or othersurface side of the porous substrate and can exhaust the hydrogen fromthe other surface or one surface 5 side. Thus, when the hydrogenseparator 1 is used for separation of hydrogen from to-be-treated gas,it is possible that the to-be-treated gas is taken in from the onesurface 5 side and hydrogen is exhausted from the other surface side, orthe to-be-treated gas is taken in from the other surface side andhydrogen is exhausted from the one surface 5 side.

In the hydrogen separator 1 of the present embodiment, thehydrogen-separating layer 3 is formed on the one surface 5 of the poroussubstrate 2 via an intermediate layer 10. This intermediate layer 10 ismade of an electron-conductive ceramic and does not prevent the hydrogenflow from the other surface side or the one surface 5 side. As aspecific example of the electron-conductive ceramic constituting theintermediate layer 10, there can be mentioned at least one memberselected from the group consisting of titanium nitride (TiN), zirconiumnitride (ZrN), indium-tin oxide (In₂O₃.Sn) (hereinafter, sometimesreferred to also as “ITO”), zinc oxide (ZnO) and titanium oxide[TiO_(x)(0<x<2)]. The electron-conductive ceramic has electronconductivity and accordingly has good affinity with thehydrogen-separating layer 3 made of a metal (a metal component).Further, the electron-conductive ceramic is a kind of ceramic andaccordingly has good affinity also with the porous substrate composedmainly of a ceramic. Thus, the hydrogen separator 1 of the presentembodiment has a layer made of an electron-conductive ceramic, as theintermediate layer 10 between the hydrogen-separating layer 3 and theporous substrate 2. As a result, in the hydrogen separator 1, theaffinity between the hydrogen-separating layer 3 and the poroussubstrate 2 is improved as compared with a case when thehydrogen-separating layer 3 and the porous substrate 2 are contacted indirect. Consequently, the hydrogen separator 1 of the present embodimenthardly generates defects such as peeling, cracks or the like in thehydrogen-separating layer 3 and is suitable for use even when thehydrogen separator 1 is exposed to a heat cycle, used under hightemperature conditions or/and used for long-term.

The porous substrate 2 usable for the hydrogen separator 1 is composedmainly of a ceramic, which has heat resistance, corrosion resistance andhigh mechanical strengths. There is no particular restriction on thekind of the ceramic, and any ceramic ordinarily used in hydrogenseparators can be employed. There can be mentioned, for example, atleast one member selected from the group consisting of alumina, silica,silica-alumina, mullite, cordierite and zirconia. Incidentally, theporous substrate 2 may contain small amounts of components presentinevitably and components added ordinarily. Furthermore, the poroussubstrate 2 may form any shape as far as it can be used as a substratefor hydrogen separator without causing any problem during the actual useas a hydrogen separator. For example, it may be a disc, plate, tubewhich may have one hole or more, or honeycomb structure.

The porous substrate 2 has a large number of pores connectingthree-dimensionally. The diameters of the pores are between 0.003 and 2μm preferably, and between 0.1 and 1 μm more preferably. When the porediameters are less than 0.003 μm, a gas flow may be somewhat prevented.When the pore diameters are more than 2 μm, it is difficult to plug thepores of porous substrate with a hydrogen-separating layer 3, and gastightness of hydrogen separator 1 may become worse.

The pores of the porous substrate 2 are preferred to be uniform indiameter. With the uniformity of pores in diameter, there can be avoideda reduction of gas tightness because the pores are not plugged with ahydrogen permselective metal in formation of a hydrogen-separating layer3.

As the hydrogen permselective metal constituting the hydrogen-separatinglayer 3, there is no restriction as long as the metal allows forselective permeation of hydrogen. As such a metal, there is specificallypreferred palladium (Pd) or an alloy containing palladium (Pd).Palladium is preferred because it allows for selective and effectivepermeation of hydrogen. As the alloy containing palladium (Pd), thereare preferred an alloy between palladium (Pd) and silver (Ag) and analloy between palladium (Pd) and copper (Cu). By alloying palladium (Pd)with silver (Ag) or copper (Cu), the embrittlement of palladium (Pd) byhydrogen is prevented and efficiency of hydrogen separation can behigher at high temperatures.

The thickness of the hydrogen-separating layer 3 is preferred to bebetween 1 and 5 μm. When the thickness of the hydrogen-separating layer3 is less than 1 μm, the hydrogen-separating layer 3 may generatedefects easily; when the thickness is more than 5 μm, efficiency ofhydrogen separation in the hydrogen-separating layer 3 may reduce.

The thickness of the intermediate layer 10 is between 0.005 and 1 μmpreferably, and between 0.01 and 0.5 μm more preferably. When thethickness of the intermediate layer 10 is less than 0.005 μm, the effectof increasing the affinity between the hydrogen-separating layer 3 andthe porous substrate 2 may decrease; when the thickness is more than 1μm, the pores of the porous substrate 2 may be plugged by theintermediate layer 10. Incidentally, it is preferred that theintermediate layer 10 covers one surface 5 of the porous substrate 2 soas not to plug the pores of the porous substrate 2. Thereby, thereoccurs easily the flow of hydrogen, which has permeated thehydrogen-separating layer 3, through the pores of the porous substrate 2and the exhaust of hydrogen from other surface side of the poroussubstrate 2.

Next, an embodiment of the present process for producing a hydrogenseparator is described with reference to FIGS. 1 and 3. FIG. 3 is asectional view schematically showing a state in which an intermediatelayer has been formed on a porous substrate. In this embodiment of theprocess for producing a hydrogen separator, first, an intermediate layer10 made of an electron-conductive ceramic is formed on one surface 5 ofa porous substrate 2 composed mainly of a ceramic, as shown in FIG. 3.Then, a hydrogen-separating layer made of a hydrogen permselective metalis formed by plating on the intermediate layer 10, whereby can beobtained a hydrogen separator 1 shown in FIG. 1.

As to the method for forming the intermediate layer 10 made of anelectron-conductive ceramic on one surface 5 of the porous substrate 2,there is no particular restriction. However, it is preferred to form theintermediate layer 10 on one surface 5 of the porous substrate 2 by, forexample, sputtering, chemical vapor deposition (CVD), ion plating or thelike. With such means, the intermediate layer 10 can be uniformly formedon the whole part of one surface 5 in a desired thickness.

When there is a fear that the intermediate layer 10 is oxidized anddeteriorated, it is desired that the intermediate layer 10 is formed onthe porous substrate 2, then an anti-oxidant layer is formed on theintermediate layer 10, and the hydrogen-separating layer 3 is formed viathe anti-oxidant layer. As the metal constituting the anti-oxidantlayer, a noble metal which is resistant to oxidation is preferred.Specifically, there is preferred at least one metal selected from thegroup consisting of palladium (Pd), platinum (Pt), silver (Ag) and gold(Au).

In order to form the hydrogen-separating layer 3 on the intermediatelayer 10 by plating, it is preferred to employ, for example, chemicalplating. In forming palladium (Pd) on the intermediate layer 10 bychemical plating, first, in order to adhere an activating metal on theintermediate layer 10, the porous substrate 2 having an intermediatelayer 10 formed on one surface 5 is dipped into a solution containing anactivating metal, and washed with pure water. A divalent palladiumcompound can be preferably used as the activating metal. In order tomake the activating metal adhere on the intermediate layer 10, in casepalladium (Pd) is used as a hydrogen permselective metal, it ispreferred to dip the porous substrate 2 into an aqueous hydrochloricacid solution of palladium chloride and an aqueous hydrochloric acidsolution of tin chloride alternately.

After the activating metal has been adhered on the intermediate layer10, the intermediate layer 10 side of the porous substrate 2 is dippedinto a plating solution containing a hydrogen permeable metal, such aspalladium (Pd), and a reducing agent. Thereby, palladium (Pd) isdeposited with using the activating metal as a nucleus and ahydrogen-separating layer 3 made of palladium (Pd) is formed. As thereducing agent, there can be mentioned hydrazine, dimethylamine boran,sodium phosphinate and sodium phosphonate, or the like.

In order to form the hydrogen-separating layer 3 on the intermediatelayer 10 by plating, it is also possible to adopt electroplating usingthe intermediate layer 10 or the anti-oxidant layer as an electrode.

When an alloy of palladium (Pd) and silver (Ag) is used as the hydrogenpermselective metal constituting the hydrogen-separating layer 3, alayer made of palladium (Pd) is formed on the intermediate layer 10 bychemical plating; then, silver (Ag) is plated on the layer made ofpalladium (Pd). Thereafter, by heating of palladium (Pd) layer andsilver (Ag) layer, an alloy of palladium (Pd) and silver (Ag), which isa hydrogen permselective metal for a hydrogen-separating layer 3, isformed by mutual diffusion. Incidentally, in plating silver (Ag) on thelayer made of palladium (Pd), chemical plating or electroplating, inwhich the layer made of palladium (Pd) is used as an electrode, ispreferred. In this case, the mass ratio (Pd:Ag) of palladium (Pd) andsilver (Ag) used is between 90:10 and 70:30 preferably.

EXAMPLES

The present invention is described specifically below by way ofExamples. However, the present invention is in no way restricted tothese Examples.

Example 1

A disc-shaped α-alumina porous substrate, which has an outer diameter of30 mm, a thickness of 3 mm and an average surface pore diameter of 0.2μm, was prepared as a porous substrate. A 100 nm-thick intermediatelayer made of titanium nitrate (TiN) and a 100 nm-thick anti-oxidantlayer made of palladium (Pd) were formed on the α-alumina poroussubstrate by sputtering. The α-alumina porous substrate having anintermediate layer was washed with pure water and then subjected to anactivation treatment. The activation treatment was carried out bydipping the α-alumina porous substrate in a solution containing divalentpalladium (Pd) compound and then by a reducing treatment. The α-aluminaporous substrate after the activation treatment was dipped into asolution containing a palladium (Pd) salt, a complexing agent and areducing agent, to prepare palladium (Pd) layer on the intermediatelayer by chemical plating. A hydrogen-separating layer made of palladium(Pd) with a thickness of 2 μm was formed and a hydrogen separator ofExample 1 was obtained.

The hydrogen separator obtained was heat-treated in argon (Ar) gas at700° C. for 1 hour (hereinafter, this heat-treatment is referred to as“700° C. heat-treatment”). A one-cycle heat-treatment operationconsisting of (1) a temperature increase from room temperature to 500°C. in nitrogen (N₂) gas under a pressure of 100 kPa, (2) aheat-treatment in hydrogen (H₂) gas under a pressure of 100 kPa at 500°C. for 1 hour, and (3) a temperature decrease from 500° C. to roomtemperature in nitrogen (N₂) gas under a pressure of 100 kPa, wasexecuted 50 times; that is, there was executed a 50-cycle heat-treatment(hereinafter, referred to as “500° C.-50 times heat-treatment”).

Helium (He) gas leakage amount of the hydrogen-separating layer made ofpalladium (Pd) was measured before the 700° C. heat treatment, after the700° C. heat-treatment, and after the 500° C.-50 times heat-treatment.The helium (He) gas leakage amount was measured by introducing a helium(He) gas into the hydrogen-separating layer side of the α-alumina poroussubstrate at a pressure of 800 kPa and measuring the amount of helium(He) gas leaking from other surface side of the α-alumina poroussubstrate. A change in helium (He) gas leakage amount is shown in Table1.

Example 2

A hydrogen separator of Example 2 was obtained in the same manner as inExample 1 except that a 100 nm-thick intermediate layer made ofzirconium nitride (ZrN) was used in place of titanium nitride (TiN) onan α-alumina porous substrate by sputtering. The hydrogen separatorobtained was treated with the same heat-treatments as in Example 1 andmeasured for helium (He) gas leakage amount. A change in helium (He) gasleakage amount is shown in Table 1.

Example 3

A hydrogen separator of Example 3 was obtained in the same manner as inExample 1 except that a 100 nm-thick intermediate layer made of anindium-tin oxide (ITO) was used in place of titanium nitride (TiN) on anα-alumina porous substrate by sputtering and a hydrogen-separating layerwas formed thereon directly without formation of anti-oxidant layer. Thehydrogen separator obtained was treated with the same heat treatments asin Example 1 and measured for helium (He) gas leakage amount. A changein helium (He) gas leakage amount is shown in Table 1.

Comparative Example 1

A hydrogen separator of Comparative Example 1 was obtained in the samemanner as in Example 1 except that a hydrogen-separating layer wasformed directly on an α-alumina porous substrate and neitherintermediate layer made of titanium nitride (TiN) nor anti-oxidant layermade of palladium (Pd) was formed on the α-alumina porous substrate. Thehydrogen separator obtained was treated with the same heat treatments asin Example 1 and measured for helium (He) gas leakage amount. A changein helium (He) gas leakage amount is shown in Table 1.

TABLE 1 Helium (He) gas leakage amount [ml/(cm² · min)] Inter- Before700° C. After 700° C. After 500° C.- mediate heat heat 50 times heatlayer treatment treatment treatment Example 1 TiN 0.0053 0.0055 0.0056Example 2 ZrN 0.0049 0.0056 0.0060 Example 3 ITO 0.0061 0.0062 0.0063Comparative — 0.0055 0.0881 0.2052 Example 1

As is clear from Table 1, the hydrogen separators of Examples 1 to 3 aresubstantially low in a reduction of gas tightness after 700° C.heat-treatment or after 500° C.-50 times heat-treatment as compared withComparative Example 1. Incidentally, the same effects as in Examples 1to 3 were confirmed also when there was used, as an intermediate layer,zinc oxide (ZnO) or a titanium oxide [TiO_(x) (0<x<2)].

INDUSTRIAL APPLICABILITY

The hydrogen separator of the present invention hardly generates defectssuch as cracks or the like in the hydrogen-separating layer, is suitablefor use under high temperature conditions or/and for long-term use, andis useful as a separator for selectively separating only hydrogen from ahydrogen-containing gas such as steam-reformed gas or the like.

1. A hydrogen separator comprising: a porous substrate composed mainlyof a ceramic having a large number of pores communicating from onesurface of the substrate to other surface, and a hydrogen-separatinglayer made of a hydrogen permselective metal formed on the poroussubstrate, via an intermediate layer made of an electron-conductiveceramic which is zinc oxide, wherein the intermediate layer has athickness between 0.01 μm and 0.5 μm, and the hydrogen-separating layerhas a thickness between 1 μm to 5 μm, wherein the intermediate layer isdirectly bonded to the porous substrate so as not to fill the pores ofthe porous substrate.
 2. A hydrogen separator according to claim 1,wherein the ceramic for a porous substrate is at least one memberselected from the group consisting of alumina, silica, silica-alumina,mullite, cordierite and zirconia.
 3. A hydrogen separator according toclaim 1, wherein the hydrogen permselective metal is palladium (Pd) oran alloy containing palladium (Pd).
 4. A hydrogen separator according toclaim 1, wherein an anti-oxidant layer is formed on the intermediatelayer in an inert atmosphere or under a vacuum condition and thehydrogen-separating layer is formed via the anti-oxidant layer.
 5. Ahydrogen separator according to claim 4, wherein the anti-oxidant layeris a layer made of at least one metal selected from the group consistingof palladium (Pd), platinum (Pt), silver (Ag) and gold (Au).
 6. Ahydrogen separator according to claim 1, wherein the pores in the poroussubstrate are between 0.003 μm and 2 μm.
 7. A hydrogen gas separatoraccording to claim 1, further comprising an anti-oxidant layer formedbetween the intermediate layer and the hydrogen-separating layer.
 8. Ahydrogen gas separator according to claim 1, wherein the hydrogen gasseparator is heat treated in argon gas at a temperature of 700° C. forone hour and a helium gas leakage amount after heat treatment varies byno more than 0.0007 ml/cm²min from a helium gas leakage amount beforeheat treatment.
 9. A process for producing a hydrogen separatorcomprising: providing a porous substrate composed mainly of a ceramichaving a large number of pores connecting from one surface of thesubstrate to other surface; forming an intermediate layer made of anelectron-conductive ceramic, which is zinc oxide, on the one surface ofthe porous substrate, wherein the intermediate layer is directly bondedto the porous substrate so as not to fill the pores of the poroussubstrate; forming a hydrogen-separating layer on the surface of theintermediate layer, wherein the hydrogen-separating layer is made of ahydrogen permeselective metal; wherein the intermediate layer has athickness between 0.01 μm and 0.5 μm, and the hydrogen-separating layerhas a thickness between 1 μm and 5 μm.
 10. A process for producing ahydrogen separator according to claim 9, wherein the ceramic for aporous substrate is at least one member selected from the groupconsisting of alumina, silica, silica-alumina, mullite, cordierite andzirconia.
 11. A process for producing a hydrogen separator according toclaim 9, wherein the hydrogen permselective metal is palladium (Pd) oran alloy containing palladium (Pd).
 12. A process for producing ahydrogen separator according to claim 9, wherein an anti-oxidant layeris further formed on the intermediate layer in an inert atmosphere orunder a vacuum condition and the hydrogen-separating layer is formed viathe anti-oxidant layer.
 13. A process for producing a hydrogen separatoraccording to claim 12, wherein the anti-oxidant layer is a layer made ofat least one metal selected from the group consisting of palladium (Pd),platinum (Pt), silver (Ag) and gold (Au).